One approach of producing white light is to mix the light from several colored light sources (e.g., light emitting diodes (LEDs) or tri-phosphor florescent lamps) to create a spectral power distribution (SPD) that appears white. By locating, for example, red, green and blue light sources adjacent to one another, and properly mixing the amount of their outputs, the resulting light can be white in appearance. With the addition of LED light sources to the palette of lighting options for lighting designers, the challenge of getting color right in a project is getting harder. Many designers have experienced this the hard way, with either very disappointing initial installations, or installations failing over time, and having to implement painful fixes. Getting color right is not easy, but it can be done, and involves obtaining and checking the right specifications from the suppliers, and getting a good understanding of the principles of measuring and matching color. It is thus a challenge to match color and to effectively render color without distortion.
The color, or more precisely, the chromaticity, of white light sources falls into the vicinity of a slightly curved line in the CIE chromaticity diagram, called Planckian locus. This curve represents the chromaticity of the light emitted by an ideal black body when it is heated, and is similar to the light generated by an iron rod forged by a blacksmith, or a tungsten filament in a light bulb heated by the current flowing through the filament. The chromaticities of these are in general close to the Planckian locus, and are commonly denoted by the temperature of the black body closest in chromaticity in CIE 1960 chromaticity diagram. This temperature is called correlated color temperature (CCT).
Light-emitting diodes (LEDs) are typically binned by a manufacturer according to output intensity and peak wavelength. However, variations in both output intensity and peak wavelength occur between LEDs in the same bin. For a mixed system that includes multiple channels of multiple white or non-white LEDs that are driven at unique flux levels to produce a combined white light emission, the variations inherent in state of the art binning of the LEDs remains too large. Although the color point of the binning tends to be relatively tight (e.g., 3-step or worse, typically), the luminosity of the LEDs per unit driving current varies substantially more. As a result, the color points for a mixed LED system, which depend on relative channel luminosity as well as color point, routinely deviate three to six MacAdam ellipse steps. Furthermore, the mixed system channel ratios for optimal color metrics, such as color rendering index (CRI), color quality scale (CQS), and R9, may vary even more with variations in peak wavelength and phosphor emission profile. The same CRI may require a different blend of channel weights (e.g., different flux/lumens values from different colored light sources) if the spectra from each light source (e.g., LED) are slightly different. Consequently, the output of an LED-based lamp with LEDs from the same bin can vary dramatically. There is thus a need to develop a way to model each LED-based lamp accurately such that the output color and intensity of the lamp is to be predictably adjusted.